The present invention is related to antifuses in integrated circuits and, more particularly, to methods of programming antifuses.
Antifuses are found in a growing number of integrated circuits, most of which are field programmable gate arrays (FPGAs). As the name implies, antifuses have a very high resistance (to form essentially an open circuit) in the unprogrammed ("off") state, and a very low resistance (to form essentially a closed circuit) in the programmed ("on") state. In these integrated circuits antifuses are placed at the intersections of interconnection lines which lead to different elements of the integrated circuit. By programming selected antifuses, the interconnections between the various elements of the integrated circuit are formed to define the function of the device.
In a typical antifuse structure a nonconducting programming layer is sandwiched between two metal interconnection lines. To program an antifuse, a large programming voltage is placed across the antifuse so that the programming layer is ruptured and a programming current passes between the two metal interconnection lines. The result is that the programmed antifuse has a conducting link formed between the two metal interconnection lines.
Nonetheless, there are problems which have been observed in the resistance values, R.sub.ON, of the programmed antifuse. One problem is that the resistance of the programmed antifuse can vary considerably. This is not desirable since widely varying resistances in the signal paths, i.e., the metal interconnection lines, cause timing problems in the programmed FPGA integrated circuit. Furthermore, high resistances in the programmed antifuses slow the performance of the device.
Another problem is reliability. The R.sub.ON of some antifuses have been found to drift even when the integrated circuit is not in use. Not only do timing problems arise in the programmed FPGA, but also the resistance of the nominally programmed antifuse might rise so high so as to be considered an open circuit. This is disastrous since the desired circuit is no longer realized in the programmed integrated circuit.
While antifuse manufacturers have found some solutions to these problems in the design of the antifuse structure itself, other solutions have been found in the manner in which the antifuse is programmed. For instance, some manufacturers have found it efficacious to reverse the flow of programming currents through the antifuse. However, polarity reversals in the programming current requires complex, and correspondingly expensive, programming circuits.
The present invention solves or substantially mitigates these problems without complex programming circuits.